A stack mold is an arrangement wherein two or more sets of molds are interspersed in sequence between a stationary platen and a movable platen of an injection molding machine. Each mold set comprises a core part and a cavity part, referred to as a "level", hence the term "four level" when four such mold sets are "stacked" between the machine platens.
The mold sets are moved between a closed molding arrangement wherein they are held together to an open or stripping arrangement wherein they are separated to enable removal or stripping of molded articles formed therebetween. The motive force for mold opening and closing is provided by the movable platen of the injection molding machine.
Reference is now made to FIG. 1A which schematically illustrates a typical prior art four level stack mold arrangement generally indicated by reference 10. The arrangement 10 includes first, second, third and fourth mold levels 12, 14, 16 and 18 respectively. Each of the mold levels 12, 14, 16 and 18 includes a cavity part 20 and a core part 22. The cavity parts 20 and core parts 22 are mounted on five sets of mold support plates which are identified in FIG. 1 as first, second, third, fourth and fifth mold support plates 24, 26, 28, 30 and 32 respectively. The first mold support plate is attached to a fixed platen 34 and the fifth mold support plate is secured to a movable platen 36. The expression "mold support plate" is used herein in a broad sense to include arrangements wherein the mold support plates are actually "sets" of plates rather than individual plates.
Only the fifth mold support plate 32 receives motive force from the movable platen 36. As all of the mold levels 12, 14, 16 and 18 need to be separated simultaneously by equal amounts, distribution apparatus is required to distribute movement of the movable platen 36 evenly to all four mold levels. One such apparatus is the prior art centering arrangement illustrated in FIG. 1 which comprises first, second and third centering devices 38, 40 and 42 respectively.
FIG. 1B is a partially cut away perspective view illustrating a typical centering device, generally identified by reference 50, of the type used for the first, second and third centering devices, 38, 40 and 42 respectively referred to above. As the centering devices have similar components, for the sake of simplicity, like reference numerals are applied to like components and the following general description applies to any of the centering devices referred to herein.
The centering device 50 has an intermediate connector 52 which would be rigidly secured to one of the second third or fourth mold support plates 26, 28 or 30 respectively. The centering device 50 further includes a shaft 54 which is journalled in the intermediate connector 52, the latter typically including a bearing, in such a manner that the shaft 54 is rotatable about a shaft axis 56 in the directions of arrows 58 but constrained against relative axial movement along the shaft axis 56. In other words, the shaft 54 can rotate in the intermediate connector 52, but cannot slide longitudinally through it.
The shaft 54 is provided with respective splines 60 and 62 extending from an intermediate portion 63 toward opposite ends of the shaft 54. The splines 60 and 62 are not parallel to the shaft axis 56, but are helically arranged about the shaft axis 56 with the helical arrangement of the splines 60 twisting opposite to the helical arrangement of the splines 62. The expression "splines" is being used herein to refer to a pattern of "grooves" extending along the surface of the shaft 54 rather than to a shaft having such a pattern. In other words, the shaft 54 would preferably be a single shaft with grooves twisting in opposite respective directions toward each end and not two grooved shafts connected end to end (which is not to say that the latter arrangement couldn't be made workable).
The splines 60 and 62 are received in corresponding splines in respective end connectors 64 and 66. The end connector 66 is shown partially cut away in FIG. 1B to illustrate splines 68. The end connectors 64 and 66 are rigidly secured to respective mold support plates, as discussed in more detail below.
In the arrangement illustrated in FIG. 1A, the second centering device 40 has its intermediate connector 52 secured to the third mold support plate 28 and one of its end connectors 64 and 66 secured respectively to each of the first mold support plate 24 and fifth mold support plate 32. Accordingly, as the moveable platen 36 is moved toward the right (away from the fixed platen 34) as illustrated in FIG. 1B, the shaft 54 will be caused, by the end connectors 64 and 66, to rotate and simultaneously "unthread" therefrom. The intermediate connector 52 permits the shaft 54 to rotate and allow the "unthreading" from the end connectors 64 and 66. As the intermediate connector 52 will not allow relative axial movement between the shaft 54 and the intermediate connector 52, the shaft 54 moves axially with the intermediate connector 52 along the shaft axis 56.
The helical splined arrangement prevents axial movement of either of the end connectors 64 and 66 without simultaneous rotation of the shaft 54 about the shaft axis 56. The shaft 54 however cannot rotate without simultaneous axial movement through the other of the end connectors 64 and 66. The pitch of the helical splines 60 and would typically be the same and accordingly, the end connectors 64 and 66 will move toward or away from the intermediate connector 52 in opposite but equal amounts in response to movement of the moveable platen 36. The rigid securement of the end connector 64, intermediate connector 52 and end connector 66 to the first, third and fifth mold support plates 24, 28 and 32 respectively will therefore maintain the third mold plate 28 equidistant from the first mold plate 24 and third mold plate 32 as the mold is opened and closed, hence the term "centering device".
In an analogous manner, the first centering device 38 has its intermediate connector 52 secured to the second mold support plate 26 and the end connectors 64 and 66 respectively secured to the first mold support plate 24 and third mold support plate 28. The third centering device 42 has its end connector 64, intermediate connector 52 and end connector 66 respectively secured to the third, fourth and fifth mold support plates 28, 30 and 32 respectively.
The first centering device 38 acts to center the second mold support plate 26 between the first and third mold support plates 24 and 28 respectively. The third centering device 42 acts to center the fourth mold support plate 30 between the third and the fifth mold support plates 28 and 32 respectively. The total effect of the first, second and third centering devices 38, 40 and 42 is to cause all four mold levels, 12, 14, 16 and 18 respectively to open and close simultaneously at generally the same rate and amount.
The first, second and third centering devices 38, 40 and 42 respectively, would typically be duplicated on the opposite side of the mold arrangement 10 to ensure even force distribution thereby avoiding cocking and subsequent jamming. Also, the first centering device 38 and third centering device 42 would typically be mounted behind the second centering device 40, horizontally in line with a mold axis 70. The second, third and fourth mold support plates 26, 28 and 30 respectively would typically be slidably supported on suitable guide rods, however this ancillary structure has not been illustrated to avoid cluttering the illustrations. Such support arrangements are well known to persons skilled in such molds.
Although the FIG. 1A arrangement provides satisfactory operation, it has two different lengths of shaft 54 and the maximum distance travelled and rate of travel between the intermediate connector 52 and the end connectors 64 and 66 is twice that for the second centering device 40 than it is for the first or third centering devices 38 and 42 respectively. Also, the prior art arrangement cannot be readily adapted for a different number of mold levels, such as a three level stack mold.
It is an object of the present invention to provide a centering arrangement for a stack mold having three or more levels which uses substantially identical centering devices to provide improved part interchangeability, to reduce the number of different parts required and to provide an arrangement wherein part wear is similar for each centering device. It is a further object of the present invention to provide a centering arrangement which can be replicated for stack molds having three or more levels.